Many photonics applications require the use of optical radiation detectors. Examples are optical radar, monitoring of laser power levels for materials processing, and laser metrology. Different types of optical detectors are available, covering the ultraviolet, visible, and infrared portions of the electromagnetic spectrum. Optical detectors convert incoming optical energy into electrical signals. The two main types of optical detectors are photon detectors and thermal detectors. Photon detectors produce one electron for each incoming photon of optical energy. The electron is then detected by the electronic circuitry. Thermal detectors convert the optical energy to heat energy, which then generates an electrical signal.
The detector circuit often employs a bias voltage and there is a load resistor in series with the detector. The incident light changes the characteristics of the detector and changes the current flowing in the circuit. The output signal is then the change in voltage drop across the load resistor.
In this module, we will describe some common optical detectors and their important characteristics. We shall not attempt to cover the entire field of light detection, which is very broad. Instead, we shall emphasize those detectors that are most commonly encountered in photonics applications.